High brightness edge-emitting semiconductor lasers are characterized by high power and low beam divergence of the output radiation and play a decisive role for a variety of applications. Standard edge-emitting semiconductor lasers comprise an epitaxial waveguide providing a tight confinement of the guided modes in sub-μm areas and hence large divergence of the output beam. Therefore the brightness of such lasers is low, additionally restricted by the power which cannot be substantially increased for small-area waveguides due to onset of the nonlinear optical effects.
High brightness edge-emitting semiconductor lasers are lasers, which are characterized by high power and low beam divergence of the output radiation. These semiconductor optical sources play an important role in a variety of applications including, for example, industrial material processing, pumping of fiber amplifiers, fiber and solid state lasers, free space communication, second harmonic generation, medicine, laser printing, lidar, etc. [1]. The emission of high brightness lasers has a slowly varying spot size across significant propagation distances (far-field zone) and is thus suitable for a large variety of direct applications, without using complicated external optical focusing units, such reducing the cost of the systems.
High brightness lasers are emitting high power and simultaneously have small divergence of the radiation in both vertical direction (direction of the epitaxial growth) and lateral direction (parallel to the epitaxial plane). The brightness B of a laser source is generally understood as the power P divided by the mode area in the focus A=πw2 (w is the radius of the beam waist) and the spatial solid angle in the far-field Θ: B=P/AΘ. For non-diffraction-limited beams the brightness is reduced by the product of the M2 factors of the beam quality for the vertical and lateral directions. The M2 factor is defined as the product of beam radius at the beam waist and the far-field beam divergence angle divided by the corresponding product for a diffraction-limited Gaussian beam with the same wavelength: M2=wΘ/(2λ/π).
Typical broad area edge-emitting lasers have a large divergence in the vertical direction (FIG. 1) and a low beam quality in the lateral direction. They consist typically of a vertical sub-μm-sized single-mode epitaxial layer waveguide of III-V or II-VI semiconductor materials and a laterally etched multi-mode waveguide of a width of several tens of μm. The far-field divergence of such lasers in the vertical direction is typically more than 50° due to tight localization of the mode at the thin waveguide layer (4) including the active region. Widening of the vertical extension of the laser waveguide as well as increasing the lateral size of the laser stripe is used to obtain an increased output of the laser. This results in an increased beam quality factor M2>>1 in both directions due to the inevitable emission of a multitude of higher order modes. Such a radiation cannot be focused to a small spot, needed for most applications, even using complicated external focusing optical systems.
Some approaches are known to decrease the beam divergence of semiconductor edge-emitting lasers in the vertical direction and to increase the brightness. Design of 650 nm GaInP/AlGaInP laser diodes was reported with two separate mode expansion layers at both sides around the active layer (FIG. 2) which have vertical far-field divergence of 24° without sacrificing threshold or threshold current temperature dependence [2]. The choice of the thickness and the refractive index (defined by the material composition) of the expansion layers is governed by the effective index of the mode, which should be close to the refractive index of these layers.
Ultra-wide waveguides based on AlGaAs/GaAs/InGaAs quantum-well heterostructures with a multitude of vertical guided modes could generate a single fundamental vertical mode at 1080 nm by asymmetric positioning of the active layers with respect to the waveguide center (FIG. 3) [3]. Asymmetric positioning results in a substantial decrease of the confinement factors for the higher order modes and provides selection of the fundamental mode. An output power of 16 W in continuous-wave operation with a vertical divergence of 31° of the vertical fundamental mode was demonstrated showing a wall-plug efficiency of 74% for a 3 mm long and 100 μm (laterally multi-mode) wide stripe, having an internal loss of as low as 0.34 cm−1.
In Ref [4] AlGaAs/GaAs/InGaAs quantum-well heterostructures were used to fabricate laser waveguides with thick expanding layer below the active layer with a refractive index close to the effective index of the fundamental mode (FIG. 4). 0.5 W power was obtained with low divergence of 11-12°. No mode selectivity was considered for the proposed thick waveguide.
The methods reported in Refs. [2-4] are suffering from growth precision issues for the expanding and active layers, when small changes of their thicknesses and/or compositions cause substantial changes of the mode selection. The same refers to temperature effects, which change the refractive index ratio of the layers and modify the modal content of the output.
A concept of a large optical cavity based on a GaAs/AlGaAs waveguide in the vertical direction with very low index contrast between active layer and waveguide layers (layers (4) and (3), (5) in FIG. 1) was proposed (see Refs. [5-6] and references therein). The low index contrast along with the asymmetric positioning of the active layer (as in FIG. 3) stimulates guiding of a broad and single fundamental mode and allows for large output power of high beam quality. This concept has resulted in ˜10 W single-mode power and a vertical beam divergence of ˜30° (95% power content) for 90 μm stripe laser [6].
Another concept for high brightness semiconductor lasers employs a thick vertical waveguide formed by a periodic multi-layered GaAs/AlGaAs sequence known as a vertical photonic bandgap crystal arrangement (FIG. 5) [7-9]. This sequence and a defect region, distorting the periodicity, were designed in a way that the fundamental mode is localized at the defect, whereas the higher order modes are extending across the entire periodic region and have small confinement factors. This approach has allowed generating a narrow vertical beam divergence down to 5° at different wavelength ranges from the visible to the near-infrared. A single mode continuous-wave output power of 2.1 W at 980 nm, and as a result, highest brightness to date for conventional semiconductor lasers of 0.35 GW/cm2sr was reported [9]. However, further increase of the power is hindered owing to high optical losses and series resistance of the lasers which are caused by the large number of the interfaces peculiar to these waveguides.
An alternative concept is based on a leaky wave laser design [9-11] providing output of the laser emission through a thick transparent substrate with extremely narrow beam divergence of <1° (FIG. 6). Development of this concept has been restrained by the existence, besides of such a low divergence beam from the substrate, also of an additional large divergence beam coming from a conventional narrow vertical waveguide including the gain medium (FIG. 6 (c)). The latter typically contains a considerable fraction of the output power and no high output power of the leaky wave lasers in the substrate mode was reported yet.